The present invention relates to an infrared sensor device with temperature correction function, and an infrared thermography for measuring the temperature of an object under measurement by detecting infrared rays emitted from the object with a two-dimensional infrared sensor. More particularly, the present invention concerns a method and an apparatus for removing adverse effects of various infrared rays attributable to the optical system on accurate temperature measurement and correcting a shading (temperature gradient) in infrared cameras using two-dimensional infrared sensors.
In the conventional infrared thermography, in order that only infrared rays emitted from the object under measurement are incident on the infrared sensor, a point sensor (i.e., a point-like infrared sensor) with a field-of-view aperture stop, which is also called cold shield and cooled to a low temperature, is used as means for detecting infrared rays. A two-dimensional thermal image is obtained by optically scanning infrared rays incident on the point sensor in the field of view with a mechanical scanner. With this mechanical scan system, it is possible to refer to a reference heat source by utilizing an ineffective period of scanning. The system corrects the output of the infrared sensor by comparing the infrared energy from the reference heat source and that from the object under measurement. By adopting this infrared sensor temperature correction method, it is possible to accurately measure the temperature of the object under measurement.
In this infrared sensor temperature correction method, the output of the infrared sensor is corrected such that the temperature represented by the output of the infrared sensor approaches the temperature of the object under measurement. Specifically, the output Q.sub.R of the infrared sensor, which is obtained when infrared rays emitted from the reference heat source at a known temperature is incident on the infrared sensor in lieu of the infrared rays emitted from the object under measurement, and the output Q.sub.W obtained when the infrared rays from the object under measurement are led through the optical system, are compared. Thereby the output component Q.sub.E of the infrared sensor which is attributable to infrared rays emitted from the other objects than the object under measurement is removed such as the optical system from the output Q.sub.W to generate an output a of the infrared sensor, which is attributable to sole infrared rays from the object under measurement.
The output a is processed in a signal processing unit to generate a thermal image signal to be displayed as a thermal image on a display. In this way, the thermal image which accurately represents the temperature of the object under measurement can be obtained.
However, in the case where a two-dimensional infrared sensor is used as infrared detecting means in the infrared thermophotograpy, the two-dimensional infrared sensor always receives infrared rays from the object under measurement. Although high accuracy can be ensured, it is impossible to refer to the reference heat source without image interruption. Besides, it is practically impossible to mount a cold shield on each of an enormous number of sensor elements. Therefore, infrared rays from objects other than the object under measurement (i.e., unnecessary infrared rays) are also received. For the above two reasons, the temperature measurement accuracy of the infrared thermographic system using the two-dimensional infrared sensor, is considerably low compared to the mechanical scan type system.
In order to improve the temperature measurement accuracy of the two-dimensional infrared sensor system, some infrared sensor temperature correction methods have been proposed. Among these methods, an estimation method, a table method and a heat source insertion method are well known in the art.
In the estimation method, a plurality of temperature sensors are disposed in an optical system to estimate by computation the unnecessary infrared energy, which is subject to variations in dependence on conditions, from the outputs of the temperature sensors. The infrared sensor temperature is corrected by subtracting the estimated unnecessary infrared energy from the infrared sensor output.
In the table method, the unnecessary infrared energy is actually measured by holding the camera part of the infrared thermographic system at various ambient temperatures, and a table showing the correspondence relation between the ambient temperature and the unnecessary infrared energy is produced for each system. When measuring the temperature of the object under measurement, the infrared sensor temperature is corrected by referring to the table in dependence on the ambient temperature.
The heat source insertion method is a temperature correction method similar to the method described before, which is adopted in the infrared thermography using a point sensor as infrared detecting means. In this system for the infrared sensor temperature correction a method is provided, which can insert a suitable reference heat source by motor driving or the like to a position right before the two-dimensional infrared sensor.
In the infrared thermography using the two-dimensional infrared sensor as infrared detecting means, some infrared sensor temperature correction methods have been proposed to improve low accuracy of temperature measurement, for example, disclosed in Japanese Laid-Open Patent Publication No. 63-163126.
In this temperature correction method, when making temperature correction, a shutter is inserted between a focusing lens and the two-dimensional infrared sensor, and also a thermistor is disposed near the shutter. For correction, the ambient temperature of the neighborhood of the shutter is measured by measuring the resistance of the thermistor, whereby the incident infrared dose from the shutter at a temperature substantially equal to the ambient temperature noted above can be known. The temperature of the object under measurement can be measured with reference to amplifier output at this time.
The above well-known methods of infrared sensor output correction, however, have respective drawbacks. In the estimation method, fluctuations with individual systems (such as sensitivity characteristics of the sensor, heat distribution in optical system and temperature measurement error of temperature sensor) cannot be absorbed. In addition, the optical system temperature distribution varies greatly with the rate of change even at the same ambient temperature. Furthermore, in the estimation method and the table method, a temperature measurement error is generated, which cannot be ignored.
Moreover, in the estimation method and the table method, it is impossible to refer to the reference heat source. Therefore, the infrared sensor is affected by electric temperature characteristics of the infrared sensor, secular changes of the optical system and so forth, and it is difficult to remove these adverse effects.
The table method has further problems that the system should be exposed to various ambient temperature conditions to obtain data, and that adjustment of the infrared thermographic system requires long time and temperature controlled vessel running expenditures.
The heat source insertion method permits referring to the reference heat source. However, the thermal image of the object under measurement cannot be obtained while the heat source is inserted. Besides, the operation of inserting the heat source with a mechanism based on a motor or the like takes long time, resulting in long thermal image interruption time. A further problem is that insertion of the heat source to be right before the infrared sensor, leads to spoiling of the equivalence of the optical system in the case when the heat source is inserted and in the other case. Therefore, even when the reference heat source is referred to, like the previous two methods, the infrared sensor output is affected by the optical system temperature distribution, and it is necessary to provide similar correction of the infrared sensor output by using a temperature sensor.
In the method disclosed in the Japanese Laid-Open Patent Publication No. 63-163126, since the shutter is inserted between the focusing lens and the two-dimensional infrared sensor, the diameter of the shutter aperture in the open state of the shutter is greater than the diameter of the conical light flux connecting the lens aperture and the focal point of the lens. This is so because the two-dimensional infrared sensor has a predetermined area (i.e., light-receiving area) so that it is necessary to cause the infrared rays from the object under measurement to be incident on the sensor area up to the edge thereof.
Therefore, in the open state of the shutter, an unnecessary infrared flux is incident from the difference area, which corresponds to the difference between the shutter aperture in the open state of the shutter and the diameter of the conical light flux noted above at the shutter position (i.e., a doughnut-like area) in addition to the infrared flux from the object under measurement on the two-dimensional infrared sensor. In the closed state of the shutter, the infrared flux from the shutter in the same field of view as when the shutter is open, is incident on the two-dimensional infrared sensor. It will be seen that the optical equivalence in the open and closed shutter states is lacking, giving rise to an error in the temperature measurement.
In an infrared camera, a phenomenon called shading (temperature gradient) arises, which has nothing to do so the object under measurement but stems from stray light in the camera casing. The shading is such that the infrared energy that is incident on a two-dimensional infrared sensor is increased as one goes from the center toward the edge of the sensor when the ambient temperature is high and reduced as one gores from the center toward the edge of the sensor when the ambient temperature is low, and it is particularly pronounced in non-cooled infrared cameras.
When such a temperature gradient is generated in spite of the fact that intrinsically it should not be present, accurate temperature measurement from infrared image cannot be obtained. Accordingly, the infrared sensor temperature is corrected by removing the effect of the shading.
In one method of such shading correction, a planar heat source at a uniform temperature is manually disposed in front of a lens, or a cap is fitted on the lens, and correction is made to make the outputs of individual image elements of the two-dimensional infrared sensor to be uniform.
In another method of shading correction, the correction is made by inserting a mechanically driven plate serving as a uniform temperature planar heat source between the lens and the two-dimensional infrared sensor.
However, the former method has a problem that a long time is required for the correction because of the manual setting of the planar heat source. The latter method has a problem that it is impossible to remove the effect of shading which is attributable to the presence of the lens.